1. Field of the Invention
The present invention is in the field of apparatus and method for depositing thin films onto substrates. More particularly, the present invention pertains to improved apparatus and method for depositing one or several thin film layers of precisely controlled thickness on one or more substrates. Still more particularly, the present invention pertains to the above-noted apparatus and method wherein the thickness of the thin film being deposited on one or more substrates is continuously monitored enabling precise control of the thickness of the film.
2. Background Art
Many optical elements have thin films coated onto them to increase their performance. Eyeglasses, windows, and camera lenses are typical of devices treated in this way. Semiconductors and medical devices are also frequently coated with optical thin films to obtain desirable surface properties. The substances which are typically deposited as thin films include but are not limited to titanium dioxide (TiO2), silicon dioxide (SiO2) and magnesium fluoride (MgF2).
Optical thin films work by interference effects which occur when a phase difference is introduced between two light waves as they are reflected at the film""s boundaries. To produce these effects the thickness of the film is usually a fraction of the wavelength of the light, but can be up to an order of magnitude thicker.
As is known in the art numerous performance enhancements are attainable through coating with optical thin films. They include increased or reduced light reflection, wavelength selection, wavelength splitting, mechanical protection, increased or reduced electrical conductivity, deposition of geometrical patterns, increase or decrease of surface tension, adhesion promotion or prevention, hermetic sealing of the surface, and others. Depending on the purpose and function of the deposited film or films, a single thin film or multiple layers up to the order of magnitude of 102 may be deposited on a substrate.
A number of different methods are employed in the art to produce optical thin films. These can be, generally speaking divided into two main categories, physical and chemical vapor deposition. Among the commonly used methods of physical vapor deposition are sputtering (attained by application of D.C. voltage, radio frequency, or in a magnetron) and evaporation (attained by resistance, electron beam, or radio frequency heating). Chemical vapor deposition commonly uses methods such as thermal decomposition, or reaction caused by plasma or active gas bleed in. Combinations of the above methods are also frequently used in the art to maximize performance and/or reduce costs. Perhaps the presently most commonly used methods for depositing optical thin films are evaporation by heating with electron beam and magnetron sputtering.
In order to attain a film deposit of the desired thickness the deposition process is usually monitored and controlled by a combination of instrumentation and devices, such as ion vacuum gauges, substrate temperature monitors, crystal thickness monitors, optical thickness monitors, uniformity masks, planetary motion controllers, electron beam vaporization controllers and others instruments and devices known in the art.
Deposition of the thin film or films is conducted in a vacuum chamber which is evacuated so that the internal pressure is in the order of magnitude of 10xe2x88x926 torr. At the low pressure prevailing in the vacuum chamber during its operation the mean free path of a particle (atom or molecule) is comparable to the distance between the source of the material to be deposited and the substrate, so that any particle moving in the direction of the substrate is likely to collide with it and be deposited thereon.
Generally speaking the state-of-the-art precision thin film coating systems fall into two categories. In one type of state-of-the-art systems the substrate is spun and also made to undergo a planetary motion (orbiting). The purpose of the planetary motion is to improve uniformity of thickness distribution of the film or films on the substrate. Because even planetary motion does not necessarily ascertain uniform thickness distribution of the film on the substrate, an also orbiting and spinning uniformity mask is also employed in the prior art to improve uniformity. As is known in the art, the shape of the uniformity mask determines its effect on the distribution of the deposited film on the substrate. In the devices employing planetary motion the thickness of the film being deposited cannot be measured directly, and therefore typically a witness sample or samples (witness chips) are placed in the center of the system. The thickness of the film being deposited on the non-orbiting witness sample can be continuously monitored by state-of-the-art instrumentation, however this measured parameter is not exactly identical with those of the film deposited on the orbiting substrate. A few prior art systems (such as the ones described in U.S. Pat. No. 4,582,431) utilize moving instruments to track a fixed location on a substrate undergoing planetary motion, but the motion of the instruments introduces additional sources of error in the precision of the deposited thin film or films.
In another type of state-of-the-art devices, the substrate spins in the vacuum chamber but does not undergo planetary motion while the optical film is deposited. In these devices the thickness of the film can be directly measured and monitored by instrumentation. Lacking the planetary motion however, the optical film is deposited less than ideally uniformly over the surface of the substrate.
Presently the most demanding performance requirements are those for Dense Wavelength Division Multiplexing Optical Elements. Manufacturing these elements requires extremely precise control of the coating process. Current coating systems are taxed to their limits when trying to create the extremely narrow bandpass optical filters needed for this type of application where band-widths of 0.8 nm, or less are typical. This usually requires control of the thickness of the deposited film with a precision approaching of 0.001 percent, or better. Typically, the deposition of 100 or more coating layers with little or no measurable deviation from their desired thickness is necessary to produce a single product. In order to reach this accuracy the use of witness sample, planetary motion, and uniformity mask is normally sacrificed in the present state-of-the-art in favor of a single optical monitor coupled to a single rotating substrate. Due to the lack of planetary motion however, this results in less than desirably uniform distribution of the films on the surface of the substrate. Yields of usable product are often quite low because many coated substrates are unusable altogether. Those that are useable generally produce only a narrow radial band of acceptable elements from within the much larger substrate.
U.S. Pat. No. 6,039,806 (Zhou et al.) discloses a precision optical coating system of this second type having a plurality of xe2x80x9cstationsxe2x80x9d in a vacuum chamber where the substrate is rotated but not orbited at each of the stations and where the deposition of the optical coating material is monitored in situ. However, even in the device of this reference the lack of planetary motion is likely to result in less then desirably uniform distribution of the coating material on the surface of the substrate.
It is also known in the art to use shutters in optical coating systems where more than one substrate is subjected to deposition of coating material. In these systems after a desired film thickness is attained on a substrate a shutter corresponding to that substrate is deployed to shield the substrate from further film deposition, while deposition of coating material on one or more substrates in the vacuum chamber is still continued. Such a shutter system is described for example in U.S. Pat. No. 6,039,806. To the best knowledge of the present inventor planetary shutters, other than shutters to cover the source, have not been used in the prior art in optical thin film depositing systems employing planetary motion.
In light of the foregoing, it is readily apparent that there is need for improvement in the state-of-the-art. There is a need to reduce the cost of manufacturing precision coated optical elements such as Rugate filters, narrow bandpass filters, dichroic filters, beam-splitters, and the like. It is also desirable to reduce the cost of manufacturing precision coated non-optical elements such as semiconductors, medical devices, sensors, and the like.
The present invention provides, for the first time known to the inventor, an optical coating system wherein optical coating material is deposited on one or more substrates where the substrate spins, undergoes planetary motion and is subjected to the uniformity enhancing effect of a corresponding uniformity mask while the thickness of the coating material can nevertheless be monitored on the substrate itself during the progress of the deposition. In one embodiment of the invention only one substrate is thin-film deposited in this manner. In a more sophisticated embodiment of the invention which employs super-planetary and sub-planetary motions a plurality of substrates are thin film-coated in the vacuum chamber while the deposited film on the substrate is monitored by appropriate instrumentation in situ. In this embodiment shutters are utilized to shield each substrate from further deposit of coating material after it has been determined that the desired thickness of film had been attained on the respective substrate.
The foregoing and other hereinafter disclosed features and advantages are attained in a device or system having a vacuum chamber, a source of the material or materials to be deposited as thin film, means for causing the material or materials to be deposited on a substrate surface, electromechanical or mechanical means for spinning and causing orbiting planetary motion to at least one substrate wherein the center of the orbiting planetary motion of the substrate is on the substrate itself, and instrumentation means for monitoring the thickness of the film of the material being deposited on the substrate itself.